Diabetes insipidus

Question 1

Diabetes insipidus vs primary polydipsia?

Answer 1

Primary polydipsia is when the patient has a normally functioning ADH system but they are drinking excessive quantities of water leading to excessive urine production

They dont have DI

Question 2

Causes of DI?

Answer 2

Nephrogenic DI

Cranial DI

Question 3

Nephrogenic DI?

Answer 3

collecting ducts of the kidneys do not respond to ADH

• Drugs, particularlylithiumused inbipolar affective disorder
• Mutations in the AVPR2 gene on the X chromosome that codes for the ADH receptor
• Intrinsic kidney disease
• Electrolyte disturbance (hypokalaemia and hypercalcaemia)

Question 4

Cranial DI?

Answer 4

hypothalamus does not produce ADH for the pituitary gland to secrete

It can be idiopathic, without a clear cause or it can be caused by:

• Brain tumours
• Head injury
• Brain malformations
• Brain infections (meningitis,encephalitisandtuberculosis)
• Brain surgery or radiotherapy

Question 5

Pathophysiology of DI

Answer 5

ID = lack of antidiuretic hormone or a lack of response toADH

• This prevents the kidneys from being able to concentrate the urine leading to polyuria (excessive amounts of urine) and polydipsia (excessive thirst). It can be classified asnephrogenic orcranial

Question 6

Clinical presentation

Answer 6

• polyuria(excessive urine production)
• Polydipsia(excessive thirst)
• Dehydration
• Postural hypotension
• Hypernatraemia

Question 7

Investigation + diagnosis of DI

Answer 7

• Low urine osmolality
• High serum osmolality

Thewater deprivation testis also known as thedesmopressin stimulation test. This is the test of choice for diagnosingDI

• Initially the patient should avoid taking in any fluids for 8 hours. This is referred to asfluid deprivation. Then,urine osmolalityis measured andsynthetic ADH(desmopressin) is administered. 8 hours later urine osmolalityis measured again.

Question 8

Cranial diabetes insipidus results of water deprivation test?

Answer 8

After deprivation: Low
After ADH: high

Incranial diabetes insipidusthe patient lacks ADH. The kidneys are still capable of responding to ADH. Therefore initially theurine osmolalityremains low as it continues to be diluted by excessive water secretion in the kidneys. Then when synthetic ADH is given the kidneys respond by reabsorbing water and concentrating the urine so the urine osmolality will be high.

Question 9

Nephrogenic diabetes Insipidus results of water deprivation test?

Answer 9

After deprivation: Low
After ADH: Low

• Innephrogenic diabetes insipidusthe patient is unable to respond to ADH. They are diluting their urine with the excessive water secretion by the kidneys. Therefore theurine osmolalitywill be low initially and remain low even after the synthetic ADH is given.

Question 10

Primary polydipsia results of water deprivation test?

Answer 10

After deprivation: high
after ADH: high

Inprimary polydipsiathe 8 hours of water deprivation will cause theurine osmolalityto be high even before the synthetic ADH is given. A high urine osmolality after 8 hours of water deprivation indicates no diabetes insipidus.

Question 11

Treatment of DI

Answer 11

If possible, treat underlying cause. Mild cases can be managed conservatively without any intervention.

Desmopressin(synthetic ADH) can be used in:

• Cranial diabetes insipidus to replace ADH
• Nephrogenic diabetes insipidus in higher doses under close monitoring